1. Field of the Invention
The present invention relates to a polling system of a terminal control unit having a polling function to a plurality of terminal devices.
2. Description of the Prior Art
In order to control data transfer among a plurality of terminal devices and a central processing unit, a terminal control unit is provided between them. The terminal control unit sequentially polls the terminal devices to inquire status of the terminal devices. That is, it scans the terminal devices. Usually, the polling is effected to all terminal device addresses which the terminal control unit processes. However, as many terminal devices as can be connected to the terminal control unit are not always actually connected to the terminal control unit. Although a terminal device address is established, a terminal device may not be connected to that address. Even if it is connected, a power supply may be kept turned off. It is waste of time to poll such a terminal device address.
The polling is disclosed in U.S. Pat. Nos. 3,517,130 and 3,341,820.